1. Field of the Invention
This invention relates generally to signal generation within an electronic article surveillance system and, more particularly, to a system and method for amplifier control within a transmitter configured to transmit signals for reception by EAS tags.
2. Description of the Related Art
In acoustomagnetic or magnetomechanical electronic article surveillance, or “EAS,” a detection system may excite an EAS tag by transmitting an electromagnetic burst at a resonance frequency of the tag. When the tag is present within the electromagnetic field created by the transmission burst, the tag begins to resonate with an acoustomagnetic or magnetomechanical response frequency that is detectable by a receiver in the detection system.
Transmitters used in these detection systems may include linear amplifiers using feedback control or switching amplifiers using open loop control. Linear amplifiers provide good transmitter current regulation with feedback control, but are expensive because of poor power efficiency, typically around forty-five percent (45%). Previous switching amplifiers provide good power efficiency, typically around eighty-five percent (85%), but transmitter current levels can fluctuate due to the open loop control and variable load conditions.
Controller components of the prior art attempt to mitigate this current fluctuation by providing a low bandwidth pulse width adjustment based on measured currents from previous transmission bursts. In one example, further described below with respect to FIGS. 1 and 2, transmitter component hardware provides a single pulse width modulator that controls a single half bridge amplifier with multiple loads connected in parallel across the amplifier output. In this configuration, the antenna with the lowest impedance receives more current than antennas with higher impedance, resulting in different levels of transmission, or power, being output from each of the antennas. Furthermore, the current sensing hardware in such prior art systems is such that only the current supplied to a single load can be sensed at any given time. Specifically, the current applied to a load is estimated after the entire transmission burst is completed by averaging the current samples.